From: avery@neuroinformatik.ruhr-uni-bochum.de (Avery Wang)
Date: Mon, 13 Jan 92 00:41:06 MET
Subject: DSP memory--paging???



OK, nobody answered my question about the DSP memory map with the
32Kword expansion module but I worked it out using the Ariel DSP
debugger (I can post it if anyone is interested).

Unfortunately it looks as if the 32K expansion memory is used, but
the 8K on-board memory is superceded.  This means that I get 32K
total DSP external memory, NOT 40K, as I had hoped... UNLESS there
is some way of paging an 8K memory subspace in and out by writing
to some IO register.  Paging has to be implemented because I saw
ads for expandability up to 576K of memory, and the addressable
space on the DSP is 64K.

Does anyone know if the 8K on-board memory is accessible at all?
Or am I stuck with only a 24K improvement?

Why is there no documentation at all on the DSP?  None came with
the package I got from my NeXT dealer.

Thanks, -Avery

 ---------               Avery Wang
                RUB-Institut fuer Neuroinformatik
                ND 03, Postfach 10 21 48
                W-4630 Bochum 1
                Germany
                awang@isl.stanford.edu or 

                avery@neuroinformatik.ruhr-uni-bochum.de
------------------------------------------------------------------------------

        DSP Memory Map for NeXT with 32K-Word Expansion Module
        ------------------------------------------------------
On-Chip         Block 1 (16K-Words)  Block 2 (16K-Words)
p:$0000-$01FF
p:              $0200-$3FFF         $4000-$7FFF     "Image A"
p:                                  $8000-$BFFF     "Image B"
p:                                  $C000-$FFFF     "Image C"

x:$0000-$00FF
x:$0100-$01FF                                         (A-law,mu-Law ROM)      

x:              $0100-$3FFF         $4000-$7FFF     "Image A"
x:                                  $8000-$BFFF     "Image B"
x:                                  $C000-$FFBF     "Image C" ($FFC0-$FFFF is 

                                                     mapped to peripheral space)

y:$0000-$00FF
y:$0100-$01FF                                        (Sine table ROM)        

y:              $0100-$3FFF         $4000-$7FFF     "Image A"
y:              $8000-$BFFF                         "Image B"
y:              $C000-$FFFF                         "Image C"

First of all, note that all memory spaces that are under Block 1 reference the
same physical memory, and similarly for Block 2.
Note that Image A is partially blocked by the on-chip RAM in locations $00-$FF 

for X- and Y-space, and $00-$1FF for P-space.  Of course you know that the 

data-ROMs in the X- and Y-space can be swapped in or out by software.
Image B is ideal for XY memory (e.g. complex data) references because the 

physical memory doesn't overlap, and the map is uninterrupted for both spaces.
Image C crashes into peripheral space, on-chip for X-space, and external 

peripheral space for Y-space, though it looks like it's OK to read/write to the
upper 64 bytes of the Y-space on the NeXT.

What are you going to do with all this memory?  Lots of DMA!  But don't get too
greedy because the maximum DMA transfer size is 8K bytes (1 vm page).

Unfortunately it looks like the expansion memory simply supercedes the on-board 

8K memory, which then becomes totally useless.  If someone knows of a magical 

software switch that can page this 8K in and out (like the data-ROMs) please
tell me!  It's a pity.  I was looking forward to 40K of DSP RAM, not 32K.